1. Field of the Invention
The present invention mainly relates to an amplifier, a radio-frequency module and a communication device for use mainly in a millimeter-wave band, or a microwave band.
2. Description of the Related Art
A related radio-frequency amplifier comprising an active element and a passive element that are mounted on slot lines is disclosed in reference (A), which is PCT Publication No. WO 97/17755. Also of background interest is reference (B), which is PCT Publication No. WO 97/17738.
In the radio-frequency amplifier in reference (A), slot lines formed on a substrate are separated in two, and the separated slot lines are used as an input unit and an output unit. The active element such as a FET is mounted on the substrate. Here, a terminal of the active element is connected to an electrode lying between terminals of the separated input unit and output unit.
The above-described radio-frequency amplifier of the related art has problems to be solved as described below.
In the radio-frequency amplifier in reference (A), the two separated slot lines are used as the input unit and the output unit of the active element, and the terminal of the active element is disposed between the separated input unit and output unit. As a result, the distance between the slots increases to at least the width of the terminal of the amplifier, thereby weakening the linkage between the slots.
For example, when the diameter of a bump of the active element to be mounted is 50 xcexcm, at least one side of a pad to be formed on the mounting substrate must be 100 xcexcm. In other words, the slot lines used as the input unit and the output unit must be separated by as much as 100 xcexcm or more.
FIG. 11 is a graph which shows simulated frequency characteristics of an FET mounting pattern in the radio-frequency amplifier of the related art.
Here, the substrate has a thickness of 0.254 mm and a dielectric constant of 10. The width of the slot is 0.1 mm, and the gap between the separated slots is 0.1 mm (100 m).
As a result, isolation deteriorates over a wide frequency band, as shown in FIG. 11. For example, when desired characteristics are obtained with xe2x88x9220 dB of insertion loss or amount of damping, the isolation band obtained has a width of only about 7 GHz. Accordingly, the number of signals that can be input to the active element decreases, while the signals which directly propagate between the input unit and the output unit increase. Therefore, the gain of the amplifier deteriorates, thereby narrowing the band in which amplifying characteristics are obtained.
The present invention has been made in view of the problems described above. The present invention provides a radio-frequency amplifier for obtaining sufficient amplifying characteristics over a wide frequency band, the radio-frequency amplifier comprising an input unit and an output unit that are formed as slot lines. The present invention further provides a radio-frequency module that comprises the radio-frequency amplifier, and a communication device that comprises the radio-frequency module.
To these ends, according to an aspect of the invention, a radio-frequency amplifier comprises a circuit substrate, a top electrode formed on the circuit substrate, an input-side slot line and an output-side slot line formed in the top electrode. Each of the input-side slot line and the output-side slot line comprises a transmitting segment and a matching segment. The radio-frequency amplifier also comprises an active element mounted on the top electrode, which comprises terminals. The active element inputs and outputs signals via the transmitting segments and the matching segments. The input-side and output-side slot lines are each bent at a given position to define the corresponding transmitting segment on one side of the given position and the corresponding matching segment, which has a length corresponding to one-quarter of the wavelength of a transmitted signal, on the other side of the given position. Each of the terminals of the active element is connected to a part of the top electrode that does not lie between the matching units. Hence, a low-loss, economical radio-frequency amplifier that operates over a wide frequency band is provided.
Preferably, the radio-frequency amplifier further comprises a DC-cut (DC-Blocking) circuit. The top electrode comprises a first electrode, a second electrode, and a third electrode. The active element is a FET that comprises a source electrode, a gate electrode, and a drain electrode. The source electrode is connected to the first electrode, which separates the slot lines. The gate electrode is connected to the second electrode, which is separated from the first electrode by the DC-cut circuit. The drain electrode is connected to the third electrode, which is separated from the source electrode and the gate electrode by the DC-cut circuit.
A radio-frequency module may comprise the above-described radio-frequency amplifier. Thus, a low-loss and high-efficiency radio-frequency module is provided.
Further, a communication device may comprise the above-described radio-frequency module. Thus, a low-loss and high-efficiency communication device is provided.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.